1. Field of the Invention
The present invention relates to a valve timing adjusting apparatus that adjusts valve timing of at least one of an intake valve and an exhaust valve of an internal combustion engine that are opened and closed by a camshaft upon transmission of a torque from a crankshaft.
2. Description of Related Art
In a known valve timing adjusting apparatus, the valve timing is adjusted by changing a relative rotational phase between two rotatable bodies, which are rotated synchronously with the crankshaft and the camshaft, respectively. For example, U.S. Pat. No. 6,637,389B2 and DE4110195C2 disclose a valve timing adjusting apparatus, which changes a relative rotational phase between two rotatable bodies through use of a differential gear mechanism, which includes a planet gear as its main component.
In the case where the differential gear mechanism is used in the valve timing adjusting apparatus, an operational frequency of the differential gear mechanism, which is operated according to the operational state of the internal combustion engine, should be extremely high. Thus, frictional wearing tends to occur in the engaged sections of the gears of the differential gear mechanism. In the apparatus of U.S. Pat. No. 6,637,389B2, the lubricant oil is guided into the interior of the apparatus through the camshaft.
Furthermore, in the apparatus of DE4110195C2, two inner gears are provided to the rotatable body synchronized with the crankshaft and the rotatable body synchronized with the camshaft, respectively, and these two inner gears are placed adjacent to each other in the axial direction. Furthermore, these inner gears are meshed with two outer gears, respectively, of the planet gear. In this way, a large speed reducing ratio can be obtained with the compact design in comparison to the apparatus disclosed in U.S. Pat. No. 6,637,389B2.
In the following, the description is made with reference to an imaginary apparatus, in which the structure of guiding the lubricant oil through the camshaft recited in U.S. Pat. No. 6,637,389B2 is provided. In this imaginary apparatus, the inner gear, which is placed far from the camshaft, has a larger diameter in comparison to the inner gear, which is placed closer to the camshaft. Thus, the lubricant oil, which has passed the space between the smaller diameter inner gear and the planet gear, receives the centrifugal force of the rotatable bodies and is thereby forced to reach the space between the larger diameter inner gear and the planet gear. Therefore, in addition to the advantages of the compact design and of the large speed reducing ratio, good lubrication can be achieved. However, in the above imaginary apparatus, the foreign material, such as abrasive debris mixed into the lubricant oil, tends to adhere to the teeth of the larger diameter inner gear among the four gears. Also, in the above imaginary apparatus, an addendum circle of the larger diameter inner gear has a smaller diameter in comparison to a dedendum circle of the smaller diameter inner gear. Thus, the teeth of these gears are axially overlapped with each other and are placed adjacent to each other. As a result, when the foreign material, which is adhered to the larger diameter inner gear, is detached from the larger diameter inner gear, the detached foreign material could enter the space between the smaller diameter inner gear and its mating outer gear and thereby be clogged there. The clogged foreign material may cause operational lock or damage in the imaginary apparatus.